This invention relates to composite structural members and particularly to electric transmission or distribution poles, and to a method of constructing such members. Electric transmission and distribution poles currently being used are, for the most part, either solid wood, hollow steel tubes or concrete, the latter being either hollow or solid. Concrete poles are further classified as being either conventionally reinforced, pre-stressed, post stressed, or partially pre-stressed. The cross-sections of the poles are either rectangular or circular. For the most part the poles are continuously tapered or, in the case of hollow metal poles, lengths of telescoping tubes of successively smaller diameter achieve cross-sectional reduction in discrete steps.
All of the conventional used poles have inherent drawbacks. Wood poles are subjected to attack by rot-producing fungi, wood-boring insects, woodpeckers, fires, and the like. Steel poles, likewise, have a number of distinct limitations. The steel poles used are generally thick-walled, heavy, and expensive since the use of weight-saving thin walled tubes of high strength steel cannot be fully realized because of the thickness limitations imposed by local buckling considerations. In addition, the interior of steel poles is inaccessible resulting in a danger of undetected corrosion. Galvanizing of the poles is often used, but it has been difficult to achieve good quality control of the interior of the poles and as a result the danger of corrosion persists. Also, steel poles are often formed from sections which are welded together and it is necessary for the welded section to develop the full tensile strength of the adjacent metal. However, quality welds are difficult to achieve and brittle fractures originating at flaws in the welds are also an ever present danger. The thickness limitation imposed by local buckling considerations also aggravates the welding difficulty by requiring welding of thick sections. Because steel poles require special manufacturing facilities, they must be fully assembled at relatively few central facilities and thus require shipping over long distances, adding significantly to the cost of the pole.
Concrete poles are also fraught with disadvantages. Conventional steel reinforced concrete poles must be designed for a bending moment substantially less than the ultimate bending moment of the pole cross-section because the concrete on the tension side will crack at relatively low tensile stresses. These cracks will admit water which, particularly in a marine environment, could cause corrosion of the steel reinforcement members. In addition, conventional steel reinforced concrete poles, because of their low design strength, are usually heavy and uneconomical to use.
Pre-stressed or post-stressed concrete poles may avoid the cracking problems of conventional steel reinforced concrete poles because the cable tensioning will close any cracks after the overload on the pole is removed. However, if overloading and crack opening is repeated too often, particles of matter may become embedded in the cracks to keep them open. Should that happen, the corrosion of the steel members may result from water entering the opened cracks.
Pre-stressed and post-stressed concrete poles are usually tapered to a smaller upper cross-section. Because the number of cables or tendons is constant along the full length of the pole, the pre- or post-stress at the lower section is less than at the top. Therefore, if the lower section of the pole is stressed the desired amount, the upper section would be overstressed. On the other hand, if the upper section is stressed the desired amount, the lower section would be under-stressed. The additional strength required at the lower section could be provided by adding conventional reinforcing steel. However, that practice could add considerably to the cost of the pole. Thus, because of the pole taper, the stressing is unbalanced between the upper and lower sections resulting in a loss of economy of the poles.
Although concrete poles require less specialized equipment for their manufacture than metal poles, the forms used are cumbersome and costly, especially where the poles are formed by centrifugal casting. Thus, a large capital expenditure is required in the forms used for producing concrete poles in order to provide a profitable production rate. In addition to the costly forms, special steam curing equipment is often required to reduce the time the forms are in use for each pole made.
There are various other disadvantages of conventional steel, wood, steel reinforced, concrete, pre-stressed concrete and post-stressed concrete poles as those skilled in this art are well aware of. The present invention overcomes most of the disadvantages enumerated above as well as those not enumerated. This invention in its various embodiments includes thin-walled metal shells and cores of non-metallic settable rigid material such as concrete embedding pre-stressed steel cables or tendons. The ability to use thin-walled casings results in cost savings in material, fabrication and shipping costs. In addition the thin-walled casing eliminate the need for special forms and rapid curing techniques. Also, the shell serves to protect the concrete core by covering the cracks which would result during overstressing of the pole while at the same time the concrete core serves to protect the interior of the steel casing from corrosion, thereby eliminating the need for costly galvanizing of the casing. The structural members of this invention also reduce the danger of weld failure as well as allowing simple inclusion of additional lower section tendons to provide balanced pre-stressed structural members. Also, the structural members of this invention may be formed in a relatively simple fabrication facility set up near the use area thereby reducing capital requirements and shipping costs as compared with the same costs of a complex factory of the nature required for forming conventionally used poles, for example. Various other advantages of this invention will be expressed at a later place in this specification.